A generally preferred form of soft switching, also referred to as lossless switching, is zero voltage switching (ZVS), in which a switching device is turned on and off at times when there is little or no voltage across the device. Another form of soft switching is zero current switching (ZCS), in which a switching device is turned on and off at times when its current during the switching is very small or zero. In each case, power losses due to the switching are relatively very small. Soft switching provides the advantages of increased efficiency and reduced power losses of the converter, and reduced electromagnetic interference (EMI). Consequently it becomes possible to operate the converter at a higher switching frequency, enabling the use of smaller components.
"A zero voltage switching flyback converter topology" by Y. Xi et al., IEEE PESC '97 Record, pages 951-957 describes a flyback converter which includes an auxiliary circuit to facilitate ZVS of the main switch of the converter. The auxiliary circuit includes a snubber capacitor in parallel with the main switch, two coupled inductors forming an auxiliary transformer for recovering energy of the snubber capacitor, and an auxiliary switch. However, in this converter energy associated with leakage inductance of the auxiliary transformer is not recovered, and turn-off of the auxiliary switch is not under ZVS or ZCS conditions, so that there are still power losses which limit the operation of the converter at very high frequencies.
Objects of this invention are to provide an improved flyback converter and an improved method of operating a flyback converter.